The widespread use of coherent soft x-ray light in numerous areas of science and technology requires the development of small-scale sources. Significant effort has been placed in the development of high repetition rate soft x-ray lasers. Discharge pumped lasers operating at 4-10 Hz repetition rate have produced milliwatts of laser average power at a wavelength of 46.9 nm [See, e.g., B. R. Benware et al., Phys. Rev. Lett. 81, 5804 (1998), and C. D. Macchieto et al., Opt. Lett. 24 1115 (1999)]. More recently, laser-pumped saturated optical field ionization lasers operating in Pd-like Xe at 41.8 nm and in Ni-like Kr at 32.8 nm have been demonstrated at repetition rates of 10 Hz using femtosecond optical laser excitation pulses of 0.33 J and 0.76 J pulses, respectively [See, e.g., S. Sebban et al., Phys. Rev. Lett. 86, 3004 (2001); and S. Sebban et al., Phys. Rev. Lett. 89, 253901 (2002)]. However, these excitation procedures have produced only saturated lasers at wavelengths above 30 nm to date. Many applications required the development of high repetition rate lasers capable of operating at shorter wavelengths.
Transient collisional electron excitation of targets using a sequence of two laser pulses impinging on the target at near-normal incidence has produced several saturated lasers in the 12-23 nm range, but required 3-10 J of pump energy, which contributed to limit operating repetition rates to only one shot every several minutes [See, e.g., P. V. Nickles et al., Phys. Rev. Lett. 78, 2748 (1997); and J. Dunn et al., Phys. Rev. Lett. 84, 4834 (2000); and K. A. Janulewicz et al., Phys. Rev. A 68, 051802 (2003).]. Where more than two laser excitation pulses impinging at normal incidence to a suitable target are used, the saturated x-ray laser gain was found to increase in some situations, and decrease in others [See, e.g., R. E. King et al., Phys. Rev. 64, 053810 (2001).].
Several excitation schemes have been investigated to reduce the necessary pumping energy and enable operation at higher repetition rates. For example, excitation of a Mo target with 150 fs, 300 mJ pulses impinging at 60° from normal incidence resulted in the appearance of the 18.9 and 22.6 nm laser lines of Ni-like Mo [See, e.g., R. Tommasini et al., Proc. of SPIE 4505, 85 (2001)], but this procedure did not produce sufficient amplification to have practical interest.
Recently, it has been demonstrated that the energy deposition efficiency of a short laser pulse can be significantly increased by directing it at grazing incidence [See, e.g., V. N. Shlyaptsev et al., Proc. of SPIE 5197, 221 (2003); and R. Keenan et al., Proc. of SPIE 5197, 213 (2003)]. In this scheme, a first laser pulse impinges on a target of selected material creating a plasma that is subsequently rapidly heated by a second pulse of picosecond duration to create a population inversion and soft x-ray laser amplification. This inherently traveling wave pumping geometry takes advantage of the refraction of the second pulse in the plasma created by the first pulse to increase its path length through the gain region of the plasma, thereby increasing the fraction of the pump energy absorbed in that region.
Accordingly, it is an object of the present invention to provide a method for increasing the output energy of soft x-ray lasers excited by grazing incidence laser pumping.
It is another object of the present invention to provide soft x-ray lasers excited by grazing incidence laser pumping having increased output energy and average power.
Additional objects, advantages and novel features of the invention will be set forth, in part, in the description that follows, and, in part, will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.